This invention relates to weather radar systems and more particularly to weather radar systems which, in addition to providing a display of precipitation rate within a field of interest, also provide a true display of turbulence or wind shear within the field of interest.
Airborne weather radars as well as most ground based meteorological radars are non-coherent pulse radars. Training guides and familiarization manuals on airborne weather radars frequently state that the primary purpose of the radar is to help the pilot to avoid turbulence associated with rainfall or other types of precipitation. However, the information displayed to the pilot is plan position of backscattering cross section, or in plain terms, of precipitation rate. The pilot is expected to supply the necessary meteorological interpretation of data displayed. Specifically, possible turbulence as well as drafts, wind shear and vorticity is inferred by the pilot from his observation of the display of precipitation rate and the gradient of such rate. The assumption is that the steeper the gradient of precipitation rate, the higher the probability that there will be accompanying turbulence, drafts, wind shear and/or vorticity. As known to those skilled in the art, the precipitation rate gradient may under certain circumstances provide an indication of turbulence or other meteorological phenomena. However, precipitation rate gradient is not a reliable indicator of such turbulence or phenomena.
One instrument currently in use for evaluating turbulence from radar return signals is the R-meter. The R-meter operates by measuring the frequency with which range gated samples of the radar detector output signal cross a threshold. The range gated signal is first boxcar detected and filtered to remove the signal components at and above the radar repetition frequency before it is compared with a threshold by a multivibrator. The threshold is normally the average value of filter output signal since this can easily be set equal to zero by making the filter a bandpass filter. The average frequency of threshold crossings, which as stated above is a measure of turbulence, is measured by conventional means. The R-meter is limited to analysis of turbulence at one range and one azimuth angle at a time.